A Question of Etiquette

So I went to pay for a DVD and fished a tenner out, which happened to be one of the older, small-portrait tens. The young woman at the check stand then looked at it closely. She rubbed it between her fingers and held it up to the lights. She looked at me suspiciously:
"So, this must be one of the old ones?" She asked as if I had just handed her a Confederate Dollar.
"Uh, yes," I replied with my usual eloquence. This didn't seem to be the right answer, as more finger rubbing and pursing of lips ensued.

I laughed, because (A) while I hadn't seen one in awhile, that bill was circa 1999, which isn't yet ancient history and so now I felt old, and (B) how is one supposed to respond to the implication that you are trying to pass counterfeit money? Anything I could think of to say seemed like the wrong answer (I am sure laughing hadn't helped), so I just stood there until she finally accepted it and I walked out.